Immediate and Delayed Impacts of Alizarin Red S Dye on Salmo trutta Fry: Physiological and Fitness Responses

The application of alizarin dye for the marking of fish is a widely adopted practice in post-stocking monitoring programmes. Nevertheless, concerns regarding the welfare implications of alizarin staining persist. The present study conclusively demonstrated that ARS dye exerts instantaneous and protracted deleterious effects on the physiological parameters (gill ventilation frequency, homeostasis in the gut microbiota, total number of erythrocytes and leukocytes) and body fitness (total length, weight and Fulton’s condition factor) of S. trutta fry. The validity of the dye-marked fish stocking effectiveness studies is called into question by these findings.